The present invention relates generally to valves utilized in subterranean wells and, in an embodiment described herein, more particularly provides a safety valve having an extension spring closure mechanism.
It is desirable for a valve utilized in a subterranean well to have a relatively thin wall thickness. This permits a larger diameter flow passage to be formed through the valve and/or permits the valve to be installed in a smaller diameter wellbore.
Most conventional safety valves use a torsion spring to bias a closure member toward a closed position relative to the flow passage. An example of a torsion spring biased closure member in a safety valve is found in U.S. Pat. No. 6,196,261, the entire disclosure of which is incorporated herein by this reference.
However, when faced with the task of reducing a safety valve's wall thickness, torsion spring closure mechanisms present several problems. For example, a torsion spring rapidly relaxes, that is, much less biasing force is produced by the torsion spring, as the closure member pivots toward the closed position. In addition, only limited space is available in the reduced wall thickness for positioning the torsion spring relative to the closure member, and a sufficiently strong torsion spring is difficult to fit into this limited space.
One solution to this problem of limited space has been to use one or more compression springs to bias the closure member to the closed position. An example of a compression spring closure mechanism in a safety valve is found in U.S. Pat. No. 6,227,299, the entire disclosure of which is incorporated herein by this reference.
However, the use of compression springs still has the disadvantage of the springs relaxing as the closure member displaces toward the closed position. In addition, the provision of the compression springs in the safety valve requires the length of the safety valve to increase, thereby increasing the cost of the safety valve. Furthermore, the compression spring mechanism requires a number of additional parts be provided in the safety valve.
From the foregoing, it can be seen that it would be quite desirable to provide an improved closure mechanism for valves, including safety valves and other types of valves, utilized in subterranean wells.